Simon Stiell said "modest steps" were taken at the talks in Bonn, Germany, but much was left undone as the world's most important climate summit looms in November in Baku, Azerbaijan.
"We took a detour on the road to Baku," Stiell, executive secretary of the UN Framework Convention on Climate Change, said at the conclusion of the meetings, seen as critical for advancing the climate agenda.
"Too many issues were left unresolved. Too many items are still on the table," he said.
Chief among them was the matter of climate finance, with nations locked in disagreement over how wealthy nations should fund clean energy and climate adaptation measures in the developing world.
The existing target of $100 billion a year expires in 2025, and a fresh goal is supposed to be agreed at COP29 in the gas-rich Caucasus nation of Azerbaijan.
But differences over how much should be raised, who should pay and receive it, and what form this money should take, threw up roadblocks during the 10-day Bonn negotiations.
Stiell said delegates would need to deliver "clear options and the substantive framework of a draft decision" if governments were to have a chance of inking a deal in November.
Success required "getting more serious about bridging divides" and not assuming a political solution could be found at the last minute in Baku, he said.
"We've left ourselves with a very steep mountain to climb to achieve ambitious outcomes," he said.
Those outside the UN climate process could also help build consensus and make headway on sticking points, he said, adding that the meeting of G7 leaders in Italy this week was "no time for resting on laurels".
Developing countries excluding China are estimated to need over $2 trillion annually by 2030 to cope with climate change.
Some wealthy countries want powerful emerging markets like China to also pay for the climate finance initiatives, while developing countries have pushed for more grants and less loans in the next round of monetary commitments.
Wealthy nations delivered $100 billion in climate finance for the first time in 2022 -- two years later than promised -- eroding trust with developing nations that are the least responsible for global warming.
"Rich developed countries talked at length about what they can't commit to and who else should pay, but failed to assure developing nations on their intent to significantly scale up financial support," Greenpeace International's Tracy Carty said in a statement.
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